Traditionally, defining a blue moon can drive a conversation crazy. I remember a heavy-dictionary definition I read years ago: "Blue moon: the length of time between recurrences of an exceedingly rare phenomenon" (dictionaries really used to talk like that). The latest and simplest definition is that a blue moon is a second full moon within one month, which means that the next Blue Moon will be tonight (just about while this is posted; been a busy day), since there was a full moon on August 1. But his definition is controversial because it's just too simple and easy to remember for something heavenly. Those in etherea seem to think should be more like the definition of Easter or Thanksgiving, which were set up back when everybody knew all about the moon and the stars, which played a big part in their nightly lives.
As so often happens therefore, if you go back in history you spoil it, which I will now do. Because historically there are multiple definitions that by the time you try to figure them out the blue moon has long gone and you couldn't care less if the moon was blue or where the name came from, just give me a single malt and make it quick.
An earlier definition holds that a blue moon is the fourth full moon in a season, when normally there would be only three, but you can see that this is pretty much the same as the two full moons in one month, which that older blue moon would have to be anyway so why not boil it down, I can't see why some spoilsport from back before there was electricity would have to muck everything up like that, when at night there was only familiar starlight and old friend moonlight, no harsh streetlights or crass neon and lots of old-fashioned time, with "up there" so important and a simple blue-moon definition right at hand; what's more, my glass is suddenly empty.
Just to liven things up a bit, here's some more confusing moon data:
Seven times in 19 years there are 13 full moons in a year.
Anyway, tonight be sure to enjoy the blue moon that isn’t blue; how did that get here?